This invention relates to inert separator beads for use with mixed-bed ion exchange resins. More particularly, it relates to spheres or beads of a crosslinked copolymer of methyl methacrylate and a hydrophilic monomer containing amide functionality, which form an inert zone separating the cation exchange resin from the anion exchange resin during regeneration of a mixed-bed resin.
Mixed-bed ion exchange resins are well known to be simple mixtures of the beads of an anion exchange resin with those of a cation exchange resin. When water or another liquid containing dissolved salt flows through a mixed-bed resin, the cation exchange beads exchange more desirable cations for the less desirable cations dissolved in the liquid, and the anion exhange beads exchange more desirable anions for the less desirable anions dissolved in the liquid. This process normally continues until the available, desirable ions of the resins have all been exchanged, whereupon these ions must be replaced by the process known as regeneration.
Cation exchange resins are normally regenerated with aqueous solutions of acids, or preferred-cation salts of acids, while anion exchange resins are normally regenerated with aqueous solutions of bases, or preferred-anion salts of the bases. Exposure of the cation resins to the cations of the anion resin regenerant, or of the anion resin to the anions of the cation resin regenerant, would effectively prevent or reverse regeneration, so the resins of a mixed bed are typically segregated for regeneration. The cation and anion exchange resins are selected with sufficiently different backwash fluidization rates, resulting from their densities and bead diameters, that classification of the resin bed by passing water upward through it causes vertical segregation of the two resins within the ion exchange columns. Regenerants are then caused to flow within their respective resins, between the top or bottom of the column and the interface of the cation exchange resin with the anion exchange resin.
It is well known to increase the thickness of this interface between the two resins by including a third, inert material in the mixed-bed resin composition which during the ion exhange treatment process is intimately mixed with the ion exchange resins, but which, because of its backwash flotation rate intermediate between those of the cation resin and the ion resin, settles between these two resins during classification prior to regeneration. As disclosed by McMullen in U.S. Pat. No. 2,666,741, this separation of the cation resin from the anion resin allows placement of the liquid inlets and outlets between them, reduces the chance of the regenerant for one resin working its way into the other resin, and allows for a reduction in bed size caused by attrition of the resins.
The separator materials, to be useful, must be neutral with respect to ion exchange properties, that is, they must be ionically neutral, having neither acidic nor basic functional ion exchange sites, and they must have a backwash fluidization rate intermediate between those of the anion exchange resin and the cation exchange resin. For practical consideration, it is helpful if the density of the separator material approaches a value intermediate between the densities of the two ion exchange resins so that the separator particle size and shape may be similar to that of the resin. The separator material should be at least as physically stable as the ion exchange resins so that shrinkage of the separation zone does not require additior of separator material before replacement of the resins. The separator material must also be resistant to attack by the influent stream and by the regenerants, which are often solutions of strong acids and strong bases. Another important criterion for the separator material is that it be wet readily by aqueous solutions.
Materials which have been used by others as inert seperator beads include beads and particles of polystyrene, polyvinyl chloride, polyethylene and hollow glass spheres. Such materials showed a tendency to aggregate with particles of the anion exchange resins, so that the backwash fluidization rates of both the resin and the separator beads were changed, and the sharpness of the classification was degraded. This problem of aggregation between the anion exchange resins and the separator beads was overcome, as disclosed by Chong et al., in U.S. Pat. No. 4,151,332, by use of a separator bead material rendered hydrophilic by inclusion of hydroxyalkyl methacrylate in its copolymer.
While the Chong et al. separator beads solved the problem of aggregation, or clumping, with the anion exchange resin beads, they left another problem unsolved, the tendency of the separator beads to cling or clump at an air-water interface, either at the surface of the liquid at the column, or around bubbles in the liquid. Under most conditions, a relatively small fraction of the separator beads cling to air-water interfaces, but all beads that do so during classification fall outside the separator layer, and to achieve the same separator thickness, an excess of separator beads must be used, reducing their efficiency as a separator. Accordingly, it is an object of the present invention to provide inert separator beads which do not cling to an air-water interface.
I have discovered inert separator beads of suitable density, which neither aggregate with anion exchange resins nor cling to air-water interfaces; these separator beads are crosslinked copolymers of methyl methacrylate, styrene, a hydrophilic monomer containing amide functionality, a polyethylenically unsaturated crosslinking monomer or mixture of such crosslinking monomers.
The inert separator resin beads of the present invention are prepared by conventional suspension polymerization techniques, employing dispersants and initiators which are well known in the art. The aqueous phase of the suspension polymerization mixture preferably contains a salt, such as sodium chloride, to reduce solubility of the hydrophilic amide monomer in water, and force it into the organic phase. This technique is again well known in suspension polymerization.
The monomers used to prepare the resin of the present invention are methyl methacrylate, a polyethylenically unsaturated crosslinking monomer, a hydrophilic monomer containing amide functionality, and styrene. The polyethylenically unsaturated crosslinking monomer or monomers include, for example, divinylbenzene, trivinylbenzene, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate and divinylnaphthalene. The preferred crosslinking monomer is divinylbenzene.
Preferred examples of the hydrophilic monomer containing amide functional groups include acrylamide, N-methyl acrylamide, N,N-dimethyl acrylamide, N-ethyl acrylamide, vinylpyrolidone, and hydroxyalkyl acrylamide wherein the alkyl group has from two to six carbon atoms, for example, N-hydroxyethyl acrylamide, N-hydroxypropyl acrylamide, N,N-dihydroxyethyl acrylamide, or N-(2,3)-dihydroxypropyl acrylamide, and the corresponding methacrylamides, namely, methacrylamide, N-methyl methacrylamide, N,N-dimethyl methacrylamide, N-ethyl methacrylamide, N-hydroxyethyl methacrylamide, N-hydroxypropyl methacrylamide, N,N-dihydroxyethyl methacrylamide, and N-(2,3)-dihydroxypropyl methacrylamide. The preferred hydrophilic monomer containing amide functional groups is methacrylamide.
The composition of the copolymer beads of the present invention is established by the amount of the monomers in the polymerization mixture. The amount of methyl methacrylate monomer may vary from about 60 to about 90%, the styrene from about 5 to about 15%, the amide monomer from about 2 to about 10% and the cross-linker from about 1 to about 6%, all percentages by weight based on the weight of the total monomer mixture. The proportions are varied such that the resulting copolymer beads have a wet density between about 1.15 and about 1.17 g/cm.sup.3. The particle size is controlled by methods well known in suspension polymerization, primarily by agitation rate, and is selected within a range from about 150 .mu.m to about 1 mm, in average diameter, preferably from about 400 to about 800 .mu.m, the specific size being chosen to allow the hydraulic density to fall midway between the hydraulic densities of the cation exchange resin and the anion exchange resin with which the separator beads will be used.
Typically, the separator beads of the present invention are mixed with beads of cation exchange resin and anion exchange resin in an amount of from about 10 to about 15% by volume, based on the total volume of the resins, although for special applications amounts greater or smaller than this may be selected. Prior to operation of the ion exchange bed for ion exchange, all three of the bead types will be mixed together, but during hydraulic classification prior to regeneration of the ion exchange resins, the ion exchange resins will be separated cleanly into two distinct vertical strata within the ion exchange column, with the separator beads lying between these two strata.
The separator beads of the present invention may be prepared by conventional suspension polymerization, using free-radical initiators, suspension aids, agitation rates, and the like, that are well known in the art. Alternatively, an expanded seed polymerization process may be used. In this process styrenic or acrylic monomers or mixture of the two are polymerized with light crosslinking to form seed beads of a size smaller than the particle size ultimately desired. These seed beads are expanded or grown, by suspending them in an emulsion of monomers including methyl methacrylate, an amide-containing monomer and a polyvinyl unsaturated, crosslinking monomer, together with a conventional emulsifier and polymerization initiator. In the expanded seed polymerization process, the preferred composition of the seeds is established by monomer fractions of from about 20 to about 75% methyl methacrylate, and more preferably from about 40 to about 60% methyl methacrylate, and a crosslinker monomer level from about 0.5 to about 1%, the balance being styrene. These monomers are polymerized in a conventional, suspension polymerization to produce a density from about 1.05 to about 1.15 g/cm.sup.3, and a particle size from about 150 to about 500 .mu.m in average diameter. The preferred monomer feed composition for the seed expansion polymerization is from about 70 to about 90% methyl methacrylate, from about 3 to about 10% hydrophilic, amide-containing monomer, preferably methacrylamide, and from about 2 to about 6% of crosslinking monomer. In both the seed formation and seed expansion polymerization, the preferred cross-linking monomer is divinylbenzene. During the seed expansion polymerization, the preferred ratio of feed monomer to seed is 4:1, and the resulting beads have a wet density of from about 1.15 to about 1.17 g/cm.sup.3, and a preferred particle diameter from about 400 to about 800.mu.m.
The following examples illustrate the invention, and not to limit it except as it is limited in the claims. All fractions are by weight, and all reagents are of good commercial quality.